This invention relates to video display of images and, more particularly, to improvements in displaying and shifting dot pattern images of characters so as to permit greater display screen bandwidth and to increase dot storage efficiency.
Whereas the invention is described herein in conjunction with a video display system incorporating a host computer and individual terminals, it is to be appreciated that the invention is not limited thereto, but may also be employed in a stand alone video display terminal or other apparatus where it is desirable to vary the positioning of dot patterns forming a character image and for minimizing the amount of storage required to provide dot patterns.
A typical video display terminal employes a TV type raster scan generator for presenting images of characters as a series of dots. Each character, may be formed within a dot matrix, such as a 7.times.9 matrix, within a somewhat larger character field matrix, such as 9.times.16. Because of subscripts or descending characters, some of the characters are positioned vertically different than others and, hence, a line of characters displayed on a screen may require much of the 9.times.16 dot matrix. Consequently, a stroke memory, typically implemented in the form of a read only memory (ROM), will require for each character upwards of 16 dot pattern strokes, even though the characters themselves may fall within a 7.times.9 dot matrix. But for such descending characters and subscripts, each character might fall within a 7.times.9 matrix and the read only memory would only need to store 9 stroke patterns for each character. Because of the necessity to provide for descending characters and subscripts, the storage requirements, then, of such a stroke generator memory is increased.